When the game stops

Shorty is really obsessive about his yellow tennis ball. He wants it all the time. He loves to chase it when we throw it, but he refuses to give it back to us; he sits down with one paw over it. Still, we throw it when we get the chance. Sometimes when it bouncing, he bumps into it. If it goes under a chair or down the stairwell at the front door, he chases it. But once in awhile it lands next to Missy. Suddenly the game stops. Shorty freezes in place, staring at the ball, looks quickly at Missy, then looks at us. Apparently we are supposed to fix this. Since he's a butt about returning the ball, we don't always fix it right away.